Micro-Electro-Mechanical System (MEMS) devices may be used in various applications such as micro-phones, accelerometers, inkjet printers, etc. A commonly used type of MEMS devices includes a capacitive sensor, which utilizes a movable element (sometimes referred to as a proof mass) as a capacitor plate, and a fixed element as the other capacitor plate. The movement of the movable element causes the change in the capacitance of the capacitor. The change in the capacitance may be converted into the change in an electrical signal, and hence the MEMS device may be used as a microphone, an accelerometer, or the like. The movement of the movable element may also be used for squeezing the ink in an inkjet printer.
The MEMS devices may include polysilicon as the capacitor plates. The capacitor plates are anchored on the underlying layers such as polysilicon layers, the silicon substrate, or the like, through anchor structures. To form the air gap that is used to allow the proof mass to move, a sacrificial oxide layer is formed, on which the proof mass is formed. The sacrificial oxide layer is then removed. An etch stop layer is accordingly formed under the sacrificial oxide layer to prevent the oxide under the sacrificial layer from being etched. It has been found that the removal of the oxide layer often results in the undesirable undercuts to be formed, wherein the undercuts may be formed under the etch stop layer. The undercuts are adjacent to where the anchor structures penetrate through the etch stop layer. The formation of the undercuts may cause the weakening of the MEMS device, and may sometimes result in the failure of the MEMS device.